Your screenshot illustrates that you have enabled the iPadOS Stage Manager feature.
With Stage Manager enabled, you will see the new Windowed UI (User Interface) - and with this you will have different controls for the windows. The black “handle” that appears in the lower corner of the illustrated App window is used to resize the related window; while Stage Manager is active, you cannot hide this element of the UI.
With Stage Manager disabled (arguably, this being the far more useful mode when using the iPad without a connected external monitor), you will see the established Full, Split and Slide-over views (accessible from the three-dot multitasking button that you'll see at top-centre of open App windows). In this mode, you will not see the re-sizing handle.
Stage Manager can be disabled (or enabled) from iPad settings:
Settings > Home Screen & Multitasking > Stage Manager > Use Stage Manager on iPad - set to OFF
The Stage Manager feature can also be simply enabled/disabled from the iPad Control Centre.
Turn Stage Manager on or off on your iPad - Apple Support